std: Tidy up some `unsafe impl`s for `sync`

This commit removes many unnecessary `unsafe impl` blocks as well as pushing the
needed implementations to the lowest level possible. I noticed that the bounds
for `RwLock` are a little off when reviewing #22574 and wanted to ensure that we
had our story straight on these implementations.

1 files changed, 2 insertions, 5 deletions

diff --git a/src/libstd/sync/once.rs b/src/libstd/sync/once.rs
index 97f985e21e8..d2054a1e819 100644
--- a/src/libstd/sync/once.rs
+++ b/src/libstd/sync/once.rs
@@ -13,10 +13,9 @@
 //! This primitive is meant to be used to run one-time initialization. An
 //! example use case would be for initializing an FFI library.
 
+use prelude::v1::*;
+
 use isize;
-use marker::Sync;
-use mem::drop;
-use ops::FnOnce;
 use sync::atomic::{AtomicIsize, Ordering, ATOMIC_ISIZE_INIT};
 use sync::{StaticMutex, MUTEX_INIT};
 
@@ -43,8 +42,6 @@ pub struct Once {
     lock_cnt: AtomicIsize,
 }
 
-unsafe impl Sync for Once {}
-
 /// Initialization value for static `Once` values.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const ONCE_INIT: Once = Once {